If I'm doing the math right, you are still only 19, is that correct? A 20 yo entering a relationship with you at 14 was an incredible disservice to you. And I'm sorry that happened.
When I have wanted to change my thought processes, I have found counseling to be invaluable! The more comfortable and confident I have become with myself, the better I am in any sort of relationship with others be it work or social or romantic.
You've received some great advice above
Hang in there!!